New advanced high strength steels by the quencing and partitioning (Q&P) process (NEWQP) : final report

The following materials and Q&P treatments were selected: CSM 2 (0.15C-3Mn-1.5Si), OCAS1 (0.25C-3Mn-1.5Si) and TKSE3 (0.28C-2.5Mn-2Si) with the corresponding Q&P cycles QT: 335ºC, PT:350ºC and Pt:100s, QT:224ºC, PT:400ºC and Pt:500s and QT: 240ºC, PT:450ºC without isothermal holding at the PT. The microstructural evolutions during partitioning revealed that more austenite was stabilized at higher PT and longer Pt and that more FM was present at higher QT. This is in the case of ideal partitioning without competing reactions. The mechanical properties, in terms of YS, UTS and TE, were ranging between 800 to 1150 MPa, 1100 to 1850 MPa and 8 to 15%, respectively. The formability and fracture analysis revealed a strong influence on the microstructure of the steels. The fatigue analysis showed that the QP steels have less fatigue sensitivity than commercial steels with similar mechanical strength. The phase model modelling (PFM) of the microstructure development was carried out. The C partitioning between the carbon supersaturated ferrite and the austenite at the QT for different partitioning condition was evaluated. Modelling of the polycrystalline microstructure of austenitic and martensitic phases by integrating Phase Field and Ginzburg-Landau approaches was carried out. The pilot-plant production parameters were evaluated and compared to process-windows in industrial production. The welding and galvanizing were successful, after some parameter optimization. The FEM simulations revealed failure in some components. The spring back behaviour was studied with the bumper geometry. Finally, the LCA showed that energy required to produce these new QP steels is comparable to that of commercially available steels.